In ionographic imaging devices such as that described by U.S. Pat. No. 4,524,371 to Sheridon et al. or U.S. Pat. No. 4,463,363 to Gundlach et al., an ion producing device generates ions to be directed past a plurality of modulation electrodes to an imaging surface in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices exist which operate similarly, but do not rely on a moving fluid stream to carry ions to a surface.
Corona efficiency in ionographic heads is very low, on the order of 0.1% to 0.5%, when efficiency is defined as the ratio of the current reaching the electroreceptor to the total current within the corona chamber. The entrainment process is inefficient because ions generated tend to follow the electric field lines, while entrainment depends upon molecular collisions. Space charge, which builds up within the ion chamber and the modulation channel, serves to limit the corona. This can be overcome by increasing air flow velocity through the head. One disadvantage of this method of improving corona efficiency is the increasing machine noise accompanying increased air flow. Dirt management and the high cost of the larger capacity air flow devices are other problems. Another method of increasing corona efficiency is to displace the coronode toward the exit channel. This utilizes geometrically non-uniform electric fields within the ion chamber to help guide the ions towards the exit channel. Thus, space charge will accumulate nonuniformly, and a greater part of the corona current entrained in the air flow will proceed through the modulation channel than with a coronode centrally located in the ion chamber. A disadvantage of this method is that corona output current is very sensitive to the coronode location and both spatial and temporal displacement from the "sweet" spot can cause variation in output current. Temporal displacement is the result of wire vibrations within the ion chamber, while spatial displacement results from tolerance stack up in the design of the wire mounting hardware.
Control of ion generation efficiency within corona devices for applying uniform charge to an imaging surface is known, for example, by U.S. Pat. No. 4,112,299 to Davis which shows a corona device with a shield formed from a plurality of conducting segments insulated from each other and biased to different potentials relative to the coronode, so that the distribution of charge from the corona device may be controlled by changing the bias arrangement, instead of requiring a device specifically designed for a particular requirement. U.S. Pat. No. 4,053,769 to Nishikawa et al. shows a corona device with a pair of insulating dielectric plates disposed in partially enclosing relationship across the corona device opening. U.S. Pat. No. 4,585,320 to Altavela et al. shows a corona device with a conductive shield grounded to increase the ion density for conduction. U.S. Pat. No. 3,742,237 to Parker suggests that an increase in the current flow from an A.C. corona device may be obtained by arranging a dielectric surface partially surrounding the coronode. U.S. Pat. No. 4,575,216 to Herbert et al. shows a dielectric ion shield extending from a point adjacent to a corona device to a point close to the entry of a paper sheet to a transfer station.